Road users increasingly rely upon traffic flow information to inform them of any incidents which may affect travel time on a journey, and to help plan travel. Such information may be provided to a user during navigation along a route via an in-car navigation device, such as a personal navigation device (PND) or integrated device, or may be provided as an input to an Advanced Driver Assistance System (ADAS). Traffic information may also be used for route planning, e.g. by a navigation device or ADAS, before commencing a journey, or to recalculate a fastest route during a journey if conditions change en route. The information has conventionally been based on messages sent over an FM radio network via the Traffic Message Channel (TMC), which may be received by navigation devices and conveyed to a user, or otherwise used by an ADAS or navigation system. A typical TMC message would include information identifying a geographic location, type and direction of an incident according to certain standard codes. More recently other traffic information systems have been developed, which are based upon so-called “probe” data obtained from mobile phones, PNDs and other devices having positioning capability located in vehicles, which can be used to identify locations and speeds of vehicles, and thus indicate traffic conditions.
The Applicant has realised that the above systems have improved the accuracy with which traffic information can be provided to users of navigation devices. However, the focus of such systems has tended to be upon route planning, e.g. allowing an optimal route to be generated taking into account traffic conditions, and/or to generate an alternative route if a planned route is affected by traffic. One aspect that has received less attention is that of using traffic information to improve safety in a road network. The presence of traffic jams affecting road segments can be a significant factor in causing accidents in the road network. The tail end of a traffic jam affecting a road segment may move at a speed substantially slower than the speed of travel along the segment that might usually be expected under non-jam collisions. Accidents may occur when vehicles approach the tail of a traffic jam at inappropriate speeds, resulting in rear-end collisions. This may occur when vehicles have inadequate warning of an upcoming traffic jam, or at least its severity, and are unable to slow down sufficiently to avoid collision when approaching the tail end of the jam.
DE 102010051244 discloses methods for generating warnings in relation to jams when a vehicle is at a predetermined distance or time from the jam tail, taking into account a position of the jam tail and speed of progression of the jam.
The Applicant has realised that there remains a need for improved methods and systems for warning drivers of traffic congestion or traffic jams affecting navigable segments in a navigable network.